1. Field of the Invention
The present invention relates generally to printing devices used in conjunction with computerized apparatus such as word processors, data processing equipment, computer terminals, etc., and more particularly to a frame and support structure used in the construction of electronic printers. The invention further relates to a plastic, resin, or thin metal type of frame structure for use in electronic printers and a technique for controlling printer platen gap spacing.
2. Related Technological Art
A side view of a printer frame commonly used in the art is illustrated for comparison purposes in FIG. 7. An exemplary frame structure is shown in FIG. 7 specifically for a printing section of a computer controlled printer. Such a printing section, as illustrated, generally uses a main guide shaft 2 mounted on a base frame 1 and positioned to extend across, or between opposite sides of, the frame, which guides a print head 3 during movement across the frame adjacent to an output medium. Print head 3 moves along guide shaft 2 and prints a pattern or matrix of dots in response to "character" codes or signals received from an external device, such as a host computer. A platen frame 5 is also attached to, or formed as part of, base frame 1 and is positioned parallel to main guide shaft 2 opposite print head 3 with a print ribbon 4 being disposed in-between. The ribbon forms the desired dot matrix pattern being printed by print head 3 onto paper 6 or another medium positioned on platen frame 5, in response to a force exerted by pins in print head 3.
In the printing section of a printer configured in this manner, it is important that the platen gap, or gap between print head 3 and platen frame 5, here labeled W, be maintained at a constant separation distance to preserve image density or visual definition of printed characters, etc., on paper 6, and to ensure that paper 6 feeds smoothly to prevent jamming. That is, if gap W is too wide, then the force applied by print head 3 against ribbon 4 and, thus, against paper 6 in forming dot matrix patterns will not be sufficient, resulting in poor print image quality due to character or dot "drop-out" or even non-printing. If, on the other hand, gap W is too narrow, paper 6 feeding does not occur at a constant speed, which results in irregular character or print spacing, and if even narrower, the paper is more prone to jamming, making printed characters unreadable.
To prevent these kinds of problems in prior art printers, base frame 1 is formed from metal or other rigid material, and platen frame 5 is rigidly fixed to base frame 1 so that a constant platen gap W and other positional relationships are maintained. Further, in situations where paper 6 thickness varies, an attachment end 7, through which main guide shaft 2 is attached to base frame 1, is generally made eccentric with respect to main guide shaft 2. The eccentricity of attachment end 7 with respect to shaft 2, allows the relative position of print head 3, as it moves along guide shaft 2, in the base frame with respect to the platen to be adjusted by angular rotation of main guide shaft 2 and, as a result, gap W is also adjusted.
However, in recent years, computer related or electronic printers have found widespread use in offices and other locations in close proximity to equipment users and other people. The relative close proximity of these printers to people makes it necessary to take measures to prevent or at least greatly decrease noise and vibration otherwise produced by the regular use of such printers. In addition, there is an increasing demand among printer manufacturers for frames that can take on a variety of shapes to improve printer compactness and to allow easier incorporation into various electronic devices and housings. While the use of vibration and sound prevention devices with printers, such as sound absorbing covers and cabinets, has been considered as one response to these demands, the use of less noisy materials for frame construction, such as plastics or resins, has also been proposed.
Unfortunately, for resin based frames, the coefficient of thermal expansion for the resin is an order of magnitude greater than that of metal, etc., with respect to linear expansion. That is, when the temperature within the printer increases due to heat generated by the drive motor and other heat generating components mounted inside the printer, the platen gap varies due to a difference between the dimensions of thermal expansion of the base frame and any metal guide elements. The coefficient of thermal expansion for a material such as steel is on the order of 0.00118 cm/.degree.C. while that of polyacetal is 0.01 cm/.degree.C. and conventional engineering plastic is 0.003 to 0.008 cm/.degree.C.
When the platen gap varies as described above, it becomes impossible to obtain clear consistent print quality, and in some cases any printing at all. Furthermore, in resin material type frames which are formed by extrusion molding, slight deformations are impossible to avoid, and if these deformations are not corrected, high quality printing cannot be achieved. Therefore, even though it is clear that many advantages could be realized by using a resin or plastic type printer frame, such frames could not previously be actually used to achieve high quality printing because of the overriding disadvantages they presented.
Another aspect of noise control, relates to the use of thinner metallic materials for constructing printer frames in order to provide more flexible or compact, and less expensive, printer designs. Unfortunately, thinner metallic materials lack sufficient rigidity and create additional noise and vibration problems, especially when used for platen portions of the printer frame. Current printer designs cannot use thinner metal parts or materials for the platen support structure without increasing the noise generated by the printer.
Therefore, a new frame design is needed that can utilize plastic or resinous materials while maintaining a constant platen gap. A new technique is also desired that would bring similar improvements in acoustical or vibration dampening when using thinner metal frames.